Implantable medical devices are commonplace today, particularly for treating cardiac dysfunction. Cardiac pacemakers, for example, are implantable medical devices that replace or supplement a heart's compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD's) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Most pacemakers today are operated in some sort of synchronous mode where the pacing pulses are delivered in a manner that is dependent upon the intrinsic depolarizations of the heart as sensed by the pacemaker. ICD's must also sense the electrical activity of the heart in order to detect an arrhythmia that will trigger delivery of the shock pulse in an attempt to reverse the condition. Such sensed information can be stored by the device in the form of a data log which can be transferred later to an external programmer via a radio link. Due to the limited data storage capacity in a typical implanted device, however, only a small fraction of the total sensed information is actually stored in the data log. One way to circumvent this problem would be to transmit the data log continuously or at frequent intervals to the external programmer. An implantable device has only a battery power supply, however, and the energy costs of such frequent radio transmissions would be excessive. What is needed is a low-energy data transmission method so that the implantable device can transmit logged data either continuously or at frequent intervals to an external device. It is toward this objective that the present invention is primarily directed.